There are numerous types of electrical control and indicating units that have an electric switch as an output element. In many of these types of units it is desirable to be able to check the integrity of that output switch element. If the output switch element is one which must be opened to provide the desired function there is always the risk that the switch element will be inoperable due to welding or some other problem. If the output switch element is a switch which is normally open, there is the problem of whether a circuit element connected to the switch may inadvertently have become open circuited.
In the case of normally closed output switches it has been common practice to electrically check the continuity of the switch and make the assumption that if the switch shows continuity that the switch element is in proper operating condition. There is no way of determining whether the switch has become inadvertently welded or is otherwise inoperable in a closed position. In the case of a normally open output switch a check of continuity that checks for an open circuit may also be unreliable. If the circuitry connected to switch inadvertently has become open circuited, the true status of the output switch is not detectable. In many types of safety equipment, such as burner control equipment, it is essential that the true nature of the output control switch be reliably determined. In certain types of safety related equipment, redundant switches are used to improve reliability, but this becomes an expensive expedient that may also be subject to the same problems as were noted above.